Time sidereal time

Now that #1145 is (nearly) in, calculating sidereal time becomes simple.  With this PR,
```
from astropy.time import Time
t=Time(['2013-01-01', '2013-01-02', '2013-01-03'], scale='utc')
t.gmst
```
yields
```
<Longitude [u'6h43m13.99092s' u'6h47m10.54534s' u'6h51m07.09960s']>
```

This uses `Gmst06` from `erfa`, but there are quite a few other methods, relying on different precession models. Before continuing, I hoped to get some input on what sofa methods should be exposed (and on whether my implementation in `Time` is a good one). For greenwich apparent sidereal time, there are even more methods, since one also has different nutation models (and, for full precision, one might need the IERS tables again). 

p.s.  I'll rebase once #1145 is merged, so there is less to look at; for now, just check https://github.com/mhvk/astropy/commit/bb406382161364b67cbb0f4734c14203d95f352a